%%% EX 6
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ok

%%% EX 7
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two minor errors:
- expanded should be initialized to {}
- node should be added to expanded

%%% EX 8
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
not solved

%%% EX 9
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
a) not solved
b) not solved
c) not solved
d) not solved

%%% EX 10
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
a little messy, but ok